Ionic liquids are a class of fused salt compositions that are liquid at low temperatures, with melting points often below room temperature. In general, such compositions have found applications as catalysts, solvents and electrolytes. Metal halide ionic liquid catalysts are attractive in many refinery process applications where the liquid catalyst is easily mixed with the reactants in a reactor, and readily separated from hydrocarbon products in a settler.
As a result of their use in catalytic reactions, ionic liquids may become partially inactivated or spent. Following catalyst deactivation, catalytic activity can be maintained by draining a quantity of the partially spent catalyst and adding a relatively large volume of fresh ionic liquid as make-up material. However, due to the expense of ionic liquids, the volume of fresh ionic liquid make-up material should be minimized on economic grounds.
Methods for the regeneration of ionic liquid catalysts by treatment with a regeneration metal are disclosed in U.S. Pat. No. 7,674,739 to Elomari, et al. A consequence of such treatment is that excess metal halide may accumulate in the ionic liquid. U.S. Pat. No. 7,674,739 further discloses the removal of excess metal halide from an ionic liquid by the addition of either an organic halide salt or a mixed salt, corresponding to the ionic liquid, having a metal halide/salt molar ratio less than two.
During the regeneration of, for example, a spent chloroaluminate ionic liquid catalyst using aluminum metal, aluminum trichloride is produced as part of the regeneration chemistry. Using N-butylpyridinium heptachlorodialuminate as an example of an ionic liquid catalyst, the target AlCl3/N-butylpyridinium chloride molar ratio is two. In this case, the “excess” aluminum trichloride formed during catalyst regeneration can be accommodated by the addition of N-butylpyridinium chloride (a hygroscopic solid).
The use of the salt precursor (e.g., N-butylpyridinium chloride) as make-up material, as opposed to using the ionic liquid itself (N-butylpyridinium heptachlorodialuminate), would be advantageous in requiring a lesser amount of make-up material. However, it is generally preferred to use liquid feeds rather than solids in large scale continuous processes operating under pressure. Handling a hygroscopic salt, such as N-butylpyridinium chloride, would be particularly problematic.
A salt such as N-butylpyridinium chloride may be liquefied by adding a little water to provide a relatively benign aqueous solution. However, an aqueous solution of the salt precursor is unsuitable as make-up material due to the need to avoid the introduction of water into the plant. Drying of aqueous salt solutions typically involves precipitation of the solid salt, the handling of which would be undesirable. Consequently the challenge of converting the aqueous solution to the anhydrous salt on-site by a convenient and preferably continuous method remains.
There is a need for methods for adjusting and maintaining the catalyst inventory of ionic liquid catalyzed processes for the steady state operation of such processes. There is a further need for preparing make-up material for amending ionic liquid catalyst inventory in a cost-effective and convenient manner.